Digital input modules are used in programmable logic controllers, motor drivers, and other circuits to convert high voltage digital inputs from field sensors and switches into digital logic signals, which are provided to other circuits. In response to the input voltage being greater than a triggering voltage, the digital logic signal is logic high. In response to the input voltage being less than the triggering voltage, the digital logic signal is logic low. Many digital input modules include a current limiter to reduce current drawn from the voltage inputs and reduce power dissipation through the digital input module.
Some digital input modules are not protected against fault conditions arising from short circuits within the digital input module. In the event of a short circuit, current within the digital input module increases and may result in overcurrent and overheating conditions, causing the digital input module to malfunction. In some cases, the digital input module malfunction causes secondary effects in other circuits configured to receive the digital logic signal output by the digital input module. Some digital input modules cannot detect device malfunctions from short circuits, which causes the digital logic signal output by the digital input module to carry incorrect information to other circuits configured to receive it. This causes the other circuits to operate under mistaken information regarding the voltage input to the digital input module.